The present invention relates to concave gratings in which grooves are formed while changing groove spacings according to a predetermined function.
Among the monochrometers which have been used for the spectroscopic analyses of ultraviolet rays, visible rays and near infrared rays, the Seya-Namioka monochromator, as disclosed in the Journal of the Optical Society of America, Volume 49, No. 10, October, 1959, pp. 951-961, features of a very simple construction employing a concave grating having grooves which are formed in the concave surface. This device covers a wide spectral region and has been extensively used for vacuum ultraviolet applications, as well as visible and ultraviolet applications. The Seya-Namioka monochromator, however, presents a greater abberation during the focusing operation than monochromators which employ plane gratings or those other monochromators which employ concave gratings.